Currently, direct-current (DC)-to-direct-current (DC) converters are becoming increasingly common in today's society as the capabilities and use of such DC-to-DC converters continue to expand. DC-to-DC converters are typically employed to convert one DC voltage level to another desirable DC voltage level. The DC-to-DC converters can be widely used in a variety of environments. One kind of such converters is referred to as constant on time converter, also known as pulse-frequency modulated (PFM) converter. Another kind of such converters is referred to as fixed frequency converter, also known as pulse-width modulated (PWM) converter. A PWM converter usually is divided into two categories, a voltage-mode converter and a current-mode converter.
A voltage-mode converter includes a control loop that contains an error amplifier, a PWM comparator, and one or more drivers. Usually a synchronous rectifier is included in the voltage-mode converter to improve performance. The output voltage of the voltage-mode converter is compared with a reference voltage by the error amplifier. The PWM comparator receives the output of the error amplifier as its first input and receives a saw-tooth or triangle signal from an external device as its second input. The PWM comparator's output is a PWM signal that is amplified by the drivers and the driver will drive power switches. The advantage of this kind of converters is its simplicity in architecture. Its major disadvantages are low precision and slow response to transient loads because of the compensation needed for the error amplifier.
A current-mode converter includes two control loops, an inner current loop and an outer voltage loop that controls the inner current loop. With reference to FIG. 1, a prior art current-mode boost converter 100 is illustrated. The boost converter 100 is composed of an inductor 110, a power switch 120, a voltage divider 130, an error amplifier 140, a compensation unit 150, an amplifier 160, an adder with an internal feedback loop 170, a comparator 180, and a driver 190. The inductor 110 is coupled to an external load (not shown) via a diode 102 and a capacitor 103. The inductor 110 receives an input voltage VIN from an external source (not shown). An output voltage VOUT larger than VIN can be supplied by the boost converter 100 to power the external load. When the power switch 120 is turned on, a current can flow through a resistor 101 and convert to a voltage signal. The voltage signal with a component of the current is then delivered to the amplifier 160 and amplified with a factor, for example, 6. The amplified voltage signal will be added to a ramp signal from an oscillator (not shown) and the adder 170 generates a sum signal.
The voltage divider 130 can scale down the output voltage VOUT and deliver a feedback voltage to the error amplifier 140. The error amplifier 140 compares the feedback voltage with a reference voltage and generates an error signal to the comparator 180. The comparator 180 compares the error signal with the sum signal from the adder 170 and generates a PWM signal to the driver 190. The driver 190 converts the PWM signal to a control signal to drive the power switch 120. The compensation unit 150 provides frequency compensation so as to regulate the output voltage VOUT.
FIG. 2 illustrates a block diagram of a prior art current-mode buck converter 200. The buck converter 200 is configured similar to the boost converter 100, so the symbols for the similar components are consistent. Hence, the description of the functions for these similar components of the buck converter 200 will be omitted herein for clarity. The buck converter 200 includes a power switch 220. The buck converter 200 can provide an output voltage VOUT smaller than the input voltage VIN.
The insertion of the amplifier 160 or 260 may have a certain bandwidth requirement and also result in signal distortion, slow transient response and large limitation on the switching frequency of the power switch 120 or 220. Moreover, a feedback loop included in the adder 170 or 270 used to improve the stability of the converters has bandwidth requirements that also cause a great limitation on the switching frequency of the power switch 120 or 220. Additionally, the instability for D (duty cycle)>0.5 is a well-known problem in the IC design. Therefore, a ramp signal added to the current flowing through the power switch 120 or 220 is required for the converter 100 or 200 to maintain its output signal stable for all duty cycles. However, the addition of the ramp signal has an effect on reducing the again of the inner switch-current-sensing discrete feedback loop formed by the amplifier 160 and the adder 170. Hence, the major disadvantages of the conventional current-mode converter 100 or 200 are the complexity of circuitry configuration and limited switching frequency, for example, less than 1 MHz.
It is thus desirous to have an apparatus and method that can provide a current-mode DC-to-DC converter with a simplified configuration, high precision and good stability that operates when the switching frequency of a power switch is high and at the same time improve the transient response of the current-mode DC-to-DC converter, and it is to such apparatus and method the present invention is primarily directed.